TO CLEAN OR NOT TO CLEAN...THAT IS THE QUESTION

Cleaning Solar Panels Often Not Worth the Cost

Don't hire someone to wash your dirty solar panels. That's the conclusion of a study recently conducted.

Researchers found panels that hadn't been cleaned, or rained on, for 145 days during a summer drought in California, lost only 7.4 percent of their efficiency. 

Overall, for a typical residential solar system of 5 kilowatts, washing panels halfway through the summer would translate into a mere $20 gain in electricity production. For larger commercial rooftop systems, the financial losses are bigger but still rarely enough to warrant the cost of washing the panels. On average, panels lost a little less than 0.05 percent of their overall efficiency per day. The study is focused on smaller systems, for very large installations, economies of scale may mean that washing panels is worth it.

Dust on PV panels does make a difference but it's not a big enough factor in Wisconsin to warrant cleaning. Researchers believe that this is the largest study quantifying losses of electricity output due to dirty solar panels conducted so far. Typically, particulate matter from air pollution, agriculture, construction and traffic accumulates on the panels, as well as pollen and sea salt.

Researchers also found that solar panels mounted at an angle of less than five degrees caused bigger losses in efficiency. That's because dirt slips off panels that are installed at a steeper angle. Here in SE Wisconsin we angle solar arrays at slightly less than 45 degrees.

But solar panels heavily soiled with bird droppings should be cleaned. That's because the droppings essentially block all sunlight and will not be washed away when it rains. Engineers also found that photovoltaic panels were dirty enough to warrant cleaning due to very specific and localized circumstances. For example, being directly next to and downwind of a highway, factory or agricultural field may generate enough dirt to warrant cleaning.

Next steps in the study would be looking more closely at the sites that did warrant cleaning and determine what caused the panels to get so dirty. Finally, researchers could add collectors at specific sites to determine what kind of dirt accumulates on the solar panels; whether special materials could keep dirt from accumulating; and whether special, less costly washing systems would do a better job at removing dirt from the panels.